The subject matter of the invention is a process for the manufacture of light colored dipropyleneglycol dibenzoate from the by-products produced in the large-scale production of dimethylterephthalate by the method of the catalytic air oxidation of p-xylene and/or methyl-p-toluate, esterification of the oxidates with methanol, and separation of the esters.
After the esterification with methanol of the acids formed in the air oxidation of p-xylene, the methyl esters -- mainly methyl-p-toluate, dimethylterephthalate and methyl esters of other mono- and polyfunctional acids -- are separated from one another by distillation or recrystallization or by a method combining these two steps. Dimethylterephthalate is obtained, useful intermediates are recycled into the process, and by-products are purged off. The by-products consist in part of methyl benzoate, which is accompanied by a number of compounds having nearly equal boiling points. Without the previous removal of these compounds whose boiling points are so close to that of methyl benzoate -- e.g., by an oxidative preliminary treatment in accordance with the method of German "Auslegeschrift" No. 1,927,544 -- the recovery of pure methyl benzoate is impossible, even with expensive distillation processes. The methyl benzoate produced in large plants from the production of dimethylterephthalate usually has a methyl benzoate content in excess of 80%.
Now, if an attempt is made to transesterify such a technical methyl benzoate containing at least 80% of this ester, which has not been pre-treated, with dipropylene glycol isomer mixture consisting of 1,1'-oxy-di-2-propanol, 2-(hydroxypropoxy)-1-propanol and 2,2'-oxy-di-1-propanol in the presence of known transesterification catalysts, it is found that some of the latter, such as antimony trioxide and boron trioxide for example, are unexpectedly completely inactive, and others, such as sodium methylate, catalyze the reaction sluggishly, indicating the formation of inhibitive substances from the substances accompanying the methyl benzoate. A number of other transesterification catalysts, such as tetrabutyl titanate, zinc compounds and magnesium compounds, are sufficiently active, but the color of the esters obtained is brown to brownish black. Iodine color numbers as high as 150 occur. In many cases the esters are so brown and even black that measurement of the iodine color numbers is not possible. A number of examples are listed in Table 1. This table shows the iodine color numbers obtained in the transesterification products of technical methyl benzoate and dipropylene glycol in identical standard batches and under comparable conditions, using a variety of catalysts. The examples are selected so as to present end products of an approximately equal degree of transesterification as indicated by their hydroxyl numbers ranging from 10 to 30.
Table 1 ______________________________________ Amount of Transester- Hy- Iodine Catalyst ification droxyl Color Catalyst (%) Time (Hours) Number Number ______________________________________ Sodium ethylate 0.2 11 24 100 Magnesium acetate 0.2 5 30 80 Magnesium 0.2 3 19 130 methylate Zinc stearate 0.2 7 13 100 Zinc dust 0.2 3 18 30 Dibutyl tin 0.2 4 17 70 laureate Tin (IV) butylate 0.2 4 blackish brown Titanium-silicon alkoxide 0.2 4 blackish brown Glycerin titanate 0.2 8 17 80 Tetrabutyl 0.2 2 27 60 titanate Tetrabutyl 0.1 4 30 60 titanate Tetrabutyl titanate 0.5 2 20 70 titanate ______________________________________
The dark color of the esters cannot be lightened substantially by means of bleaching agents such as active charcoal or bleaching earths. Oxidants such as hydrogen peroxide or ozone also produce no definite effect. Vacuum distillation yields dark yellow distillates, but the color quality is not adequate for technical use. In addition, thermolysis of the ester takes place at the high temperatures that are used, with a yielding of benzoic acid, resulting in acidity in the distillates, reductions of yield and considerable technical difficulty due to the production of sublimates in the apparatus.
The methyl benzoate that is produced in considerable quantities as a technical by-product has therefore been unable to be used for the production of light colored transesterification products.